There are numerous different applications where capacitively fed back amplifiers are required. This type of amplifier usually includes a capacitor coupled between an inverting input and the output of an operation or amplifier. Such an amplifier with a capacitive feedback requires a resistive path between the inverting input and the output of the operational amplifier in order to properly define a DC bias point of the input of the operational amplifier.
FIG. 1 shows a simplified circuit diagram of a capacitively fed back noninverting amplifier according to the prior art. There is an operational amplifier OP1 having a capacitor C2 between the inverting input and the output. There is a further capacitor C1 coupled between the inverting input and ground and a resistor R coupled in parallel to the capacitor C2 between the output and the inverting input. Accordingly, the DC operating point of the noninverting amplifier is established by resistor R. However, in integrated semiconductor circuits, the use of a resistor is not desirable in some cases due to the comparatively large area needed in order to implement the resistor and because of related parasitic capacitances.
Vittoz et al. discloses in “High Performance Crystal Oscillator Circuits”, IEEE, Journal of Solid State Circuits, page 781, FIG. 11 a capacitively fed back noninverting amplifier. However, this configuration has the disadvantage of this solution in that it requires a supply voltage of at least two threshold voltages of MOSFET transistors.